Usually, taking into consideration safety and environmental aspects, a machine tool is provided with a cover body for dividing a machining region from an external region and the cover body is provided with a window so that the machining region can be observed from the outside thereof.
As an example of such a cover body having a window, conventionally, the cover body disclosed in the Japanese Unexamined Patent Application Publication No. 3-166037 has been known. The cover body (specifically, a slide door) disclosed in this published application is made of a plate-shaped polycarbonate and has a structure in which a glass plate is attached to the inside (the machining-region side) of a portion to be a window thereof.
According to this cover body, because the polycarbonate plate constituting the cover body has very high toughness and high impact resistance, for example, even if an unexpected accident in which a tool collides with a workpiece happens due to an operation error or an error in program generation, it is possible to completely prevent the tool and the workpiece from shooting out of the machining region due to the collision.
While a polycarbonate plate has high toughness and high impact resistance as described above, its hardness is not so high, that is, its wear resistance is not so high, and therefore a polycarbonate plate has a disadvantage that, when chips generated by cutting or the like hit its surface, its surface is shaved by the chips and becomes rough, thereby resulting in deterioration in transparency (visibility) therethrough, for example.
Therefore, in the above conventional cover body, in order to prevent the surface of the polycarbonate plate from becoming rough due to chips and maintain good visibility therethrough, a structure is employed in which a glass plate having high wear resistance is attached to the inside (the machining-region side) of a portion to be a window of the polycarbonate plate.
Thus, according to the conventional cover body having the above-described structure, the glass plate arranged on the machining-region side makes it possible to prevent deterioration in visibility (good observability from the outside), which is caused by chips, and the polycarbonate plate having high toughness and high impact resistance makes it possible to completely prevent the tool and the workpiece from shooting out of the machining region, and therefore it is possible to secure high safety.
By the way, in recent years, it has been found out that the polycarbonate plate has, besides the disadvantage that it has low wear resistance, a disadvantage that it has low resistance to coolant and, if it is in contact with coolant for a long time, its high toughness and high impact resistance, which are its advantages, are reduced.
Therefore, at present, the cover body except for the window is made of sheet metal and the window employs a structure in which a glass plate and a polycarbonate plate are arranged on the machining-region side and the external side, respectively, and they are airtightly joined together with a pre-determined distance between them.
According to the window having such a structure, the polycarbonate plate is prevented from coming into contact with coolant and it is therefore possible to prevent the toughness and the impact resistance of the polycarbonate plate from being reduced due to coolant.